_ctypes fails to extend the return value per libffi contract

Bug report

Bug description:

The current implementation is relying on restype->size to write the data into a buffer, however, per libffi contract

integral types narrower than the system register size are widened

Which is not happening here. This has caused a failure on s390x when using libclang. cindex.py calls into libclang through ctypes, which builds the callback closures using libffi. With a c_int return, ctypes writes only the low 32 bits and skips the extension that both the ABI and libffi's closure contract require.

We have also opened a PR to fix this temporarily under llvm: https://github.com/llvm/llvm-project/pull/221024

CPython versions tested on:

3.11

Operating systems tested on:

Linux

Linked PRs

• gh-157045

Comienza en Modules/_ctypes/callbacks.c, en el manejo enlazado del valor de retorno, y lee el contrato de closures de libffi al que se hace referencia. Reproduce o inspecciona el comportamiento del callback c_int en s390x y, después, verifica que los valores de retorno integrales estrechos sigan la extensión ABI requerida en lugar de limitarse a copiar restype->size bytes. Comprueba el PR enlazado de CPython para ver el trabajo que ya está en curso.